The present invention relates to a method of aligning two members utilizing marks provided thereon and, more particularly, to a method of aligning a photoelectric mask and a wafer utilizing marks provided on the mask and wafer in an electron beam projecting technique.
A resist pattern can be formed on a surface of a sample, such as a silicon wafer, by scanning an electron beam of a minute size on a resist film formed on a silicon wafer to expose the resist and to form a resist pattern on the sample surface. However, according to this method, scanning the electron beam requires a long period of time so that the efficiency of forming resist patterns on wafers (wafer productivity) is low.
A mask-type transfer apparatus has been developed in order to solve this problem. In this projecting apparatus, a mask of a predetermined pattern is used, and this mask pattern is transferred onto a sample by an X-ray, an electron beam or the like. Of such projecting apparatuses, a photoelectric mask-type electron beam projecting apparatus is considered most promising for the transfer of minute patterns. According to the apparatus of this type, a magnetic field and an electric field are applied between a mask and a sample, and photoelectrons emitted from the mask upon being irradiated with ultraviolet light are focused onto the sample to transfer the mask pattern onto the sample.
In a projecting apparatus of this type, a method as shown in FIG. 1 is used so as to match the relative positions (alignment) of a photoelectric mask and a wafer. A mask 10 and a wafer 20 are arranged at a distance of about 10 mm from each other and held in a vacuum atmosphere at about 1.times.10.sup.-6 Torr. The mask 10 has a quartz substrate 12 capable of transmitting ultraviolet light, a light-shielding film 14 formed below the substrate 12 and made of a material such as chromium which is capable of shielding ultraviolet light, and a photoelectric film 16 coated below the substrate 12 and made of a material such as CsI which is capable of emitting photoelectrons upon being irradiated with ultraviolet light. A mark 18 is defined by a photoelectric material pattern replacing part of the light-shielding film 14. A mark 26 of a heavy metal such as tantalum, tungsten or molybdenum or a heavy metal compound is formed on the upper surface of the wafer 20 such as a silicon wafer. The wafer 20 is supported on a support 22. An X-ray detector 30 is arranged below the wafer 20.
The marks 18 and 26 have the same shape, for example, a rectangular shape. Assume that the wafer 20 is moved to the left in FIG. 1. The positional deviation of the wafer 20 with reference to 0 deviation obtained when the marks 18 and 26 are aligned is represented by D. Fig. 2 is a graph showing the vertical overlapping area S between the marks 18 and 26 as a function of the positional deviation D. When the length of each of the marks 18 and 26 is represented by d, the overlapping area S increases linearly as the deviation D becomes smaller than the length d and becomes at its maximum when the deviation D is zero.
When the photoelectric mask 10 is irradiated with ultraviolet light and when a magnetic field and an electric field (indicated by arrows) are applied between the mask 10 and the wafer 20 to be perpendicular to their surfaces, electron beams are generated from the mark 18 and are accelerated along these arrows. When the accelerated electron beams are bombarded against a metal target consisting of the mark 26 and the wafer 20, the electrons are decelerated upon collision with the nuclei of the target metal, and part of the kinetic energy of the electrons is emitted as X-rays. The intensity of the generated X-rays depends on the atomic number Z of the target metal, that is, the larger the Z, the larger the intensity of X-rays generated, provided that the energy of the electron beams and the thickness of the target are the same. Therefore, when electron beams are irradiated onto a metal target, the intensity of X-rays emitted from the mark 26 of a heavy metal is larger than that from the silicon wafer 20. Thus, the deviation D and the X-ray output hold the relationship as shown in FIG. 3. The wafer 20 is moved while the intensity of X-rays generated from the metal target is detected by the X-ray detector 30. When the wafer 20 is stopped at a position where the X-ray output is maximum, the marks 18 and 26 are aligned. Alternatively, the mask 10 and wafer 20 are aligned by deflecting the magnetic field applied between the mask and wafer by the magnetic field from a deflection coil by an amount corresponding to the positional deviation detected by the X-ray detector.
In this aligning method, the alignment precision is dependent on the S/N ratio of the detection signal. Since the intensity of X-rays is very small, the mark area must be increased to increase the level of the detection signal. On the other hand, the mark width must be reduced in order to improve the position detection resolution. However, when the mark width is reduced, the detection range is also reduced. In order to solve this contradictory problem, a coarse/fine alignment method using a mask pattern as shown in FIG. 4 is proposed in a document (J. P. Scott: The Electrochemical Society: (1974), p. 123). According to this method, the alignment mark consists of a coarse alignment pattern and a fine alignment pattern. Each of these patterns consists of lines and spaces which are arranged at equal intervals. When lines and spaces are arranged at equal intervals, the mark area can be increased without widening the line width. In addition to this, the position detection range can be widened by performing alignment in steps of coarse and fine alignment. Output characteristics of X-rays obtained by a method utilizing such a mark consist of a combination of the signal component from the coarse alignment and that from the fine alignment, as shown in FIG. 5. When coarse alignment is performed, the positional deviation in a coarse alignment region cannot be detected due to the influence of the signal component from the fine alignment. Therefore, it is difficult to obtain a detection range of fine alignment. A method is plausible wherein coarse and fine alignment regions are preset separately and separate detectors are arranged for the respective regions, so that coarse and fine alignments are performed with the separate detectors. However, in this case, the numbers of marks and detectors are increased to render the overall apparatus complex. Moreover, the transfer region of the wafer is reduced, resulting in a problem.
The above-identified document proposes another method of removing the influence of a signal component from a fine alignment pattern. According to this method, a current including an AC component is made to flow in a deflection coil to deflect an electron beam in an AC manner (AC deflection). Synchronous detection (lock in detection, phase sensitive detection, hereafter abbreviated as PSD) of an output signal is performed utilizing the modulated signal of an electron beam obtained by AC deflection. In this method, a rectangular wave is used as the AC wave, and the deflecting amplitude (scan width of the electron beam) is selected to be a multiple integral of the pitch of a fine alignment pattern consisting of lines and spaces. In the output obtained after this signal processing, the signal component from the fine alignment pattern is normally 0, and only the signal component from the coarse alignment pattern needs to be considered. The PSD processed output obtained in this manner is shown in FIGS. 6 and 7.
FIG. 6 shows a PSD output when the beam scan width is a multiple integral of a fine alignment pattern and is half of the pitch of the coarse alignment pattern. Meanwhile, FIG. 7 shows a PSD output when the beam scan width is a multiple integral of the pitch of the fine alignment pattern and is smaller than half of the pitch of the coarse alignment pattern. When the beam scan width is small (FIG. 7), a flat region (nonsensitive region) is present in the PSD output characteristics. In this flat region, positional deviation cannot be detected. In order to eliminate this flat region, the beam scan width must be large. However, in an electron beam transfer apparatus, it is difficult in practice to scan an electron beam within a great amplitude due to various limitations such as the coil arrangement, heat generation, and performance required for the deflection coil power supply. Therefore, it is difficult to keep the scan width of the electron beam to be below half the pitch of the coarse alignment pattern. Thus, the nonsensitive region cannot be prevented.
Another technique has been recently proposed which uses a superconductive coil as a focusing coil. According to this technique, the coil is driven in a permanent current mode to reduce the defocusing of photoelectrons due to power supply fluctuations or ripple components, and to prevent heat generation from the coil. Thus, the adverse effects of thermal deformation and temperature fluctuation are reduced. In this apparatus, when there is an AC magnetic field fluctuation, an eddy current is induced. Then, heat is generated inside the superconductive coil to evaporate liquid helium used as a coolant. The evaporation of helium is called an AC loss. When the AC loss is great, a great load is imposed on the cooler for liquefying evaporated helium. This AC loss is increased with an increase in the scan width of the electron beam. Thus, in order to reduce the AC loss, the beam cannot be AC deflected within a great amplitude.
For these reasons, an actual PSD output has a flat nonsensitive region as shown in FIG. 7. Since the positional deviation cannot be detected within this nonsensitive region, a long time period is required to control the positional deviation to a degree that the deviation falls within the range wherein fine alignment by the fine alignment pattern can be performed. The operation for this alignment is a factor in decreasing the throughput per unit time of an electron beam transfer apparatus. Furthermore, during this alignment process, the part of the wafer except for the marks may also be irradiated with an electron beam. Therefore, in order to prevent irradiation of the wafer with an electron beam during this alignment which would adversely affect the transfer of the resist pattern, the resist sensitivity and exposure time must be selected such that the radiation of the electron beam during a projection or exposure well exceeds that during alignment. In a conventional method, lengthy alignment is performed. Therefore, the exposure time must also be increased since the level of radiation for projection or exposure must be higher than that for alignment. This decreases the throughput. In this manner, in a conventional coarse/fine alignment method, coarse alignment requires a long period of time, which decreases the transfer throughput.